The present invention relates in general to gas scrubbers, and in particular, to a new and useful method for removing sulfur oxides and other contaminants contained in flue gas formed during the combustion of fossil fuels (coal, oil, petroleum coke, etc.) and/or waste materials which are burned by electric power generating plants, refuse-to-energy plants and other industrial processes.
Prior art with respect to the removal of sulfur oxides and/or other contaminants from a hot combustion exhaust gas to comply with federal and state emissions requirements as known by the inventors include:
1. Utilizing fossil fuels low in sulfur content and/or other contaminants, the major disadvantage being increased fuel and freight costs as dictated by supply/demand and proximity to the end user respectively.
2. Reduction of the sulfur content and/or other contaminants in the fuel prior to combustion via mechanical and/or chemical processes, the major disadvantage being the cost effectiveness of the mechanical and/or chemical processing necessary to meet the levels of sulfur and/or contaminants reduction required.
3. Mixing of dry alkali material with the fuel prior to combustion or injection of pulverized alkali material directly into the hot combustion gases to remove sulfur oxides and/or other contaminants via adsorption (i.e. LIMB, and Coolside Technologies as demonstrated by the Babcock & Wilcox Company and others), the major disadvantage being low to moderate removal efficiencies, poor reagent utilization and increased particulate loadings in the flue gas which may necessitate the requirements for further flue gas conditioning (i.e., humidification or sulfur trioxide injection) when the injection process is conducted upstream of an electrostatic precipitator (ESP).
4. The wet chemical adsorption process (i.e., wet scrubbing) wherein the hot gas is typically washed in an up flow gas liquid contact device with an aqueous alkaline solution or slurry to remove sulfur oxides and/or other contaminants, the major disadvantages being the loss of liquid both to the atmosphere (i.e., due to saturation of the flue gas and mist carryover) and with sludge produced in the process, the economics associated with the materials of construction for the absorber module and all related auxiliary equipment downstream (i.e., primary/secondary dewatering and waste water treatment subsystems). Babcock & Wilcox is one of several manufacturers of wet scrubbers.
5. Spray drying chemical absorption (i.e., dry scrubbing) wherein an aqueous alkaline solution or slurry is finely atomized (i.e., via mechanical, dual fluid or rotary cup type atomizers) and sprayed into a hot flue gas to remove sulfur oxides and other contaminants, with the major disadvantages being moderate-high pressure drop across the spray dryer gas inlet distribution device and the limitation on spray down temperature (i.e., approach to flue gas saturation temperature) to maintain controlled operations.
The Babcock & Wilcox Company currently has two commercial horizontal co-current flow dry scrubber systems in operation at two electric power generating plants. Babcock & Wilcox is the only known manufacturer of horizontal co-current flow dry scrubber systems for the electric utility industry. There are several major manufacturers of vertical co-current downflow dry scrubbing systems for the electric utility industry.
FIG. 1 is a perspective view, with portions removed, of a known horizontal co-current flow dry scrubber system available from the Babcock & Wilcox Company. The dry scrubber comprises a housing 10 having an inlet 12 for hot flue gas and an outlet 14 for supplying the particulate containing gases to a particulate collector. Slurry is supplied through an inlet 16 to a plurality of atomizers 18 which also receive pressurized air through an air inlet 20. A plurality of gas diffusers 22 cooperate with the outlet end of the atomizers to help mix the incoming gas flow 24 with the slurry, in a spray adsorption region 26 of housing 10. Unused slurry is recycled through the system by a flush outlet 28.
FIG. 2 is a vertical sectional view of a known vertical co-current downflow dry scrubber. The vertical scrubber comprises a housing 30 with a roof mounted gas disperser 32 for mixing the flue gas with the slurry discharged from atomizer 33.